While crossing the ocean aircraft are generally out of easy contact with ground stations for hours at a time. They're using long-range HF radios or even the data-only CPDLC system.

Some comments I've in other discussions brought up plane-to-plane communications, but other craft aren't commonly close enough to have any relevant weather information to share. (I'd extrapolate to think you'd have similar issues reliably transmitting data.)

Some fascinating comments below this post by the way: http://www.weathergraphics.com/tim/af447/